Conventionally, plating on steel materials has been widely employed, because of their susceptibility to corrosion. Zinc plating has been widely employed as plating on steel materials. To further improve the surface characteristics, the surface is activated by immersion in a nitric acid solution after the zinc plating is conducted, and then a trivalent chromium chemical conversion treatment is conducted. In this case, the steps from the zinc plating to the trivalent chromium chemical conversion treatment are often conducted continuously, and the trivalent chromium chemical conversion treatment, which is the subsequent step, may be adversely affected depending on the state of the activation treatment liquid.
The activation effects of the activation nitric acid solution are lost, because gradual accumulation of zinc ions is caused by dissolution of the zinc plating from the substrate surface due to the activation and the drag-in of the plating liquid from the zinc plating step due to attachment of the plating liquid to the product. At the time point where the accumulation of the zinc concentration in the activation nitric acid solution causes the loss of the gloss on the substrate surface after the activation treatment, the entire amount of the nitric acid solution has to be exchanged, which requires labors and the expenditure for purchasing nitric acid.
To overcome such an adverse effect, a control method and an apparatus are known in which zinc ions dissolved in an activation nitric acid solution are removed by ion exchange with hydrogen ions (Patent Document 1). However, this method requires labors for maintaining the function of the ion-exchange resin, and still has a problem in terms of economy, and hence there remains a room for improvement.